Method for determining at least one piece of information representative of a phase fraction of a fluid in a pipe

ABSTRACT

A method is presented for determining at least one piece of information representative of a phase fraction of a fluid in a pipe. The method comprises the estimation of a number of counts received in each measuring interval based on each piece of representative information determined at a preceding iteration, then calculating a residual comprising a first criterion calculated from probabilities using a given statistical law to measure, for each energy, the number of counts measured in each interval, the given statistical law being parameterized based on the number of estimated counts.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority to French Patent Application No. 1051405, filed Feb. 26, 2010.

TECHNICAL FIELD

The present invention relates to a method for determining at least one piece of information representative of a phase fraction of a multiphase fluid circulating in a pipe.

Such a method is intended for example to be implemented in a multiphase flowmeter. Such a flowmeter is in particular used to characterize the flow of a fluid extracted from a well formed in the subsoil, such as a hydrocarbon production well.

BACKGROUND ART

During the production of the well, it is known to measure the flow rate of fluid extracted from the well to be able to monitor the quantity and quality of the production. In particular, it is frequently necessary for the operator of the well to determine the overall flow rate of fluid flowing through the pipe, and if possible, the individual volume flow rates of each phase flowing in the pipe.

To determine these values, it is necessary to estimate the volume fraction of gas contained in the multiphase fluid, often referred to as “gas hold up,” and the proportion of aqueous phase present in the liquid at any moment.

To estimate these parameters, it is known to determine the relative areas occupied by each of the gas phase, the oily liquid phase and the aqueous liquid phase on a section of the pipe. To that end, a radioactive source is placed opposite a wall of the pipe to emit gamma photons at typically one or more energy levels.

The gamma photons are then oriented transversely through the fluid flowing in the pipe. A detector is placed opposite the source, opposite the pipe to collect and count the photons that pass through the multiphase fluid and determine their energy. The count number received at each energy is measured at a high frequency, for example with a sampling pitch in the vicinity of 20 ms. This makes it possible to calculate the lineic fraction of the gas, i.e. the ratio between the length of the gaseous phase crossed and the internal diameter of the pipe.

Due to the nature of the radioactive source, there is a natural statistical dispersion of the number of counts measured, even in the case where the measured fluid is static. The measurement uncertainty resulting from this dispersion can be reduced significantly by increasing the integration time.

However, in practice, the multiphase fluid circulates in the pipe at a high flow rate. This fluid is generally turbulent and sometimes has structural irregularities, for example gas bubbles in liquid, or plugs in gas that make the flow unstable.

Measuring the number of gamma photon counts remains an effective way to measure the phase fractions, regardless of the state of flow. However, in a dynamic state, the number of counts cannot be significantly averaged to reduce the statistical noise, since the nature of the flow can vary quite rapidly.

Subsequently, in certain cases, the results obtained directly based on count measurements done in a dynamic state have significant fluctuations.

To resolve this problem, U.S. Pat. No. 5,854,820 proposes a method in which the statistical fluctuations of the measured numbers of counts are compensated by an algorithm making it possible to model the attenuations and compare them to the measured attenuation. A statistical residual is calculated and is minimized to estimate the fractions.

Such a calculation increases the precision of the measurement, but can still be improved, in particular to optimize the determined values of the gas content levels and the proportion of water, in particular when these values are close to their physical boundary values.

One aim of the present disclosure is therefore to provide a method for calculating phase fractions that are more precise, even when the values of the measured fractions are close to their physical boundaries and/or it is not possible to significantly average the measured numbers of counts, given the dynamic nature of the flow.

BRIEF SUMMARY OF THE DISCLOSURE

To that end, the present disclosure relates to a method of the aforementioned type, characterized in that it comprises the following steps:

-   -   (a) emitting a number of gamma photon counts at at least one         energy through the fluid; and     -   (b) measuring a number of counts received for each energy, after         passage in the fluid;     -   (c) dividing a given measuring period into a plurality of         measuring intervals in which each piece of representative         information is assumed to be constant;     -   (d) choosing an initial value of the or each piece of         representative information in each interval;     -   (e) determining the or each piece of representative information         in each interval through successive iterations until a         convergence criterion is verified, each iteration comprising:     -   (e₁) estimating a number of counts received at each energy in         each interval based on the or each piece of representative         information determined at a preceding iteration,     -   (e₂) calculating a residual comprising a first criterion         calculated from probabilities according to a given statistical         law to measure, for each energy, the number of counts measured         at each interval, the given statistical law being parameterized         based on the number of counts estimated in step (e₁) at each         interval;     -   (e₃) determining new values of the or each piece of         representative information to minimize the residual;     -   step (e) including, at each iteration,     -   (e₄) calculating the average, for each energy, of an estimated         number of counts for each interval based on the representative         information determined in a preceding iteration, the residual         calculated in step (e₂) comprising a second criterion calculated         from probabilities according to a given statistical law to         measure, for each energy, an average number of counts, the given         statistical law being parameterized based on the average         calculated in step (e₄).

The method according to the disclosure can comprise one or several of the following features, considered alone or according to all technically possible combinations:

-   -   for each energy, the average count number is an average number         of counts in an empty pipe measured during a calibration step at         each energy, in the absence of fluid in the pipe, the step         including the calculation of the average, for each energy, of         the number of counts in an empty pipe estimated for each         interval based on the representative information determined in a         preceding iteration;     -   the representative information is respectively the lineic         fraction of gas according to a diameter of the pipe and the         volume fraction of water in the liquid phase present in the         multiphase fluid;     -   the statistical law is a Poisson law;     -   the residual is calculated using the formula:         f _(le) +f _(he)+ω·(m _(le) +m _(he))     -   in which f_(le) and f_(he) are the first criteria calculated at         the first energy and the second energy, respectively, m_(le) and         m_(he) are the second criteria calculated at the first energy         and the second energy, respectively, and ω is a weight         coefficient;     -   wherein the method comprises scanning the raw representative         information obtained at the end of step (e) at each interval to         determine the raw representative information coming from a given         interval of physical values, then blocking at least one piece of         raw representative information coming out of the interval of         physical values so that its value remains equal to one end of         the interval;

The method may further comprise a step (f) for determining each piece of representative information in each interval through successive iterations until verification of a convergence criterion, each iteration comprising:

-   -   (f₁) estimating a number of counts received at each energy in         each interval based on the representative information determined         at a preceding iteration from representative information         obtained after blocking the or each piece of locked         representative information;     -   (f₂) calculating a residual comprising a third criterion         calculated from probabilities according to a given statistical         law to measure, for each energy, the number of counts estimated         at each interval from representative information obtained         without blocking at the end of step (e), the given statistical         law being parameterized based on the number of counts estimated         in step (f₁) at each interval;     -   the blocking step includes the selection of at least a first         group of representative information having a value situated         outside the interval of physical values given, without blocking         a second group of determined representative information having a         value situated outside the interval of given physical values,         then carrying out step (f);     -   the or each piece of locked representative information is chosen         based on a criterion representative of the count transfer         resulting from blocking the representative information;     -   calculating the third criterion comprises calculating a count         transfer probability coefficient resulting from blocking at         least one piece of representative information in each interval         at each energy to weight each probability calculated in step         (f₂);     -   the step for dividing the given period into a plurality of         intervals comprises:     -   (c₁) for each interval, determining a beginning of an interval,         then calculating, for successive measuring moments moving away         from the beginning of the interval, at least one difference         between at least one statistical magnitude calculated from the         number of counts measured between the beginning of the interval         and the measuring moment after the beginning of the interval and         the same statistical magnitude calculated based on a known         statistical law, until a criterion determined based on said         difference is greater than a determined value, the measuring         moment in progress then constituting the beginning of another         interval,     -   (c₂) calculating the average of the number of counts measured at         each energy at each measuring moment (t_(r)) in the interval,         the average of the number of measured counts defining the number         of counts measured in the interval;     -   (c₃) repeating steps (c₁) and (c₂) to define the other interval         from the moment following the end of the interval.     -   the statistical magnitude comprises the variance and/or         covariance of the number of counts measured at each measuring         moment between the beginning of the interval and the subsequent         measuring moment;     -   the statistical law is a Poisson law, the statistical magnitude         being calculated from the Poisson law;     -   determining the criterion comprises calculating at least the         difference:

$\left\lbrack \frac{V_{k} - V_{k}^{0}}{\sigma_{V_{k}}} \right\rbrack$

where k is the energy, V_(k) is the variance of the number of measured counts between the beginning of the interval and the measuring moment after the beginning of the interval, V_(k) ⁰ is the variance calculated from the statistical law, and σ_(Vk) is the standard deviation calculated from the statistical law.

The disclosure also relates to a method for measuring a fluid circulating in a pipe, the method comprising the following steps:

-   -   emitting a number of gamma photon counts at at least one energy         through the fluid, and     -   measuring a number of counts received for each energy, after         passage in the fluid,     -   dividing a given measuring period into a plurality of intervals,         comprising:     -   (c₁) for each interval, determining the beginning of the         interval, then calculating, for successive measuring moments         moving away from the beginning of the interval, at least one         difference between at least one statistical magnitude calculated         from the number of counts measured between the beginning of the         interval and the measuring moment after the beginning of the         interval and the same statistical magnitude calculated based on         a known statistical law, until a criterion determined based on         said difference is greater than a determined value, the         measuring moment in progress then constituting the beginning of         another interval;     -   (c₂) calculating the average of the number of counts measured at         each energy at each measuring moment in the interval, the         average of the number of counts measured defining the number of         counts measured in the interval;     -   (c₃) repeating steps (c₁) and (c₂) to define the other interval.

This method does not necessarily comprise steps (d) and (e) above. It can comprise one or several of the above features, considered alone or according to all technically possible combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be better understood upon reading the description that follows, provided solely as an example and done in reference to the appended drawings, in which:

FIG. 1 depicts a diagrammatic view, in cross-section along a median plane of a first measuring device to implement the method according to certain embodiments of the present disclosure;

FIG. 2 depicts an illustration of the emission spectrum of the gamma photons created by the device of FIG. 1;

FIG. 3 depicts a functional synoptic diagram illustrating the main phases of the method according to certain embodiments of the present disclosure;

FIG. 4 depicts a diagrammatic view illustrating the division of the measuring period into a plurality of intervals in which the numbers of measured counts are assumed to be constant;

FIG. 5 depicts a diagrammatic view illustrating the principle of the method of dividing the measuring period into a plurality of intervals, in which each measurement is assumed to be constant;

FIG. 6 depicts a functional synoptic diagram illustrating an optimization algorithm with constraints; and

FIG. 7 depicts a view of a synoptic diagram illustrating the implementation of an algorithm for optimizing blocking of the non-physical values.

DETAILED DESCRIPTION

In all of the following, the terms “upstream” and “downstream” refer to the normal direction of circulation of a fluid in a pipe.

FIG. 1 illustrates a device 10 for measuring phase fractions of a fluid 12 circulating in a pipe 14 of a fluid exploitation installation such as a hydrocarbon production well. The fluid 12 comprises a gaseous phase, an oily liquid phase and an aqueous liquid phase.

The device 10 is intended to measure dynamically, at any moment, a first piece of information representative of the properties of the fluid, constituted by the lineic fraction of gas formed by the ratio Γ_(g) of the gaseous phase length passed through to the internal diameter of the pipe. The ratio Γ_(g) can be used to calculate the gaseous phase area fraction to the total cross-section occupied by the fluid 12, designated by the term “gas hold up” (GHU).

The device 10 is also intended to measure, at any moment, the ratio of the volume fraction of the liquid aqueous phase in the liquid phase, designated by the term W.

The device 10 is for example integrated within a multiphase flowmeter comprising a means for calculating the flow rates of fluid circulating in the pipe 14 based on fractions Γ_(g), and W measured using the device 10.

The fluid 12 circulating in the pipe 14 can have different flow regimes. In the example illustrated in FIG. 1, the fluid 12 forms an annular flow comprising an essentially liquid annular jacket 16, a gaseous core 18 circulating at the heart of the jacket 16 and drops of liquid 20 circulating in the gaseous core 18. Other flow regimes can also be measured by the device 10 according to certain embodiments of the present disclosure, such as a regime with plugs, for example.

The pipe 14 extends vertically, for example, at the outlet of a hydrocarbon exploitation installation well (not shown). The fluid 12 circulates in the pipe 14 along a substantially vertical axis A-A′ opposite the pipe.

The device 10 is for example placed in a section of the pipe 14 defining a venturi.

The device 10 includes a source 30 of gamma photon emissions, and a detector 32 for detecting receipt of the gamma photons after their passage through the fluid 12 contained in the pipe 14, the source 30 and the detector 32 being situated on either side of the pipe 14 along a diameter thereof.

The device 10 also comprises a calculation and control unit 34, and probes (not shown) for measuring the temperature and pressure in the pipe 14.

As illustrated by FIG. 2, the source 30 is capable of emitting two beams of gamma rays at different energies, i.e. a low energy (le) beam 36 of gamma photons and a high energy (he) beam 38 of gamma photons.

The number of counts per unit of time emitted by the source 30 is shown diagrammatically in FIG. 2 as a function of the energy of each photon. The photons corresponding to each energy he, le are those included in the respective energy windows he, le.

The gamma photons emitted by the source 30 pass through the fluid 12 transversely between the source 30 and the detector 32.

The detector 32 is capable of detecting, at a given sampling pitch p, the gamma photons having passed through the fluid and determining their energy. This pitch p is in the vicinity of 20 ms.

The calculation and control unit 34 is capable of measuring, at each measuring moment at measuring pitch p, the number of counts of low energy photons n_(le,p) collected at each moment by the detector 32 and the number of counts of high energy photons n_(he,p) collected at each moment by the detector 32, as a function of predefined energy windows.

The calculation unit 34 is capable of carrying out the method according to at least one embodiment of the present disclosure. To that end, it also contains a model for computing representative information Γ_(g) and W determined at each moment.

In reference to FIG. 4, the unit 34 comprises means for dividing a given measuring period T₁ into a plurality n_(t) of intervals I_(i) in which each piece of information Γ_(g,i), W_(i) is constant, a means for computing a measured number of counts n_(le,i), n_(he,i) at each energy and an estimated count number {circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i) in each interval I_(i) from the or each piece of representative information Γ_(g,i), W_(i). The unit 34 also includes a means for iterative adjustment of the value of each piece of representative information constituting a vector x=[Γ_(g,1), Γ_(g,Ng), W₁, . . . W_(Nw)] with Ng designating the number of intervals for the gas fraction and Nw the number of intervals for W. In an alternative expression, the vector x=[Γ_(g)(1), Γ_(g)(k), W(1), . . . W(i)] corresponding to the different values taken by Γ_(g) and W over the measuring period T₁ to minimize a residual, as will be described later.

In reference to FIG. 3, a first method for determining representative information Γ_(g,i), W_(i) according to the disclosure includes an initial calibration phase 100 and a measuring phase 102.

The calibration phase 100 includes a step for determining count numbers in an empty pipe measured at each energy n_(le) ⁰, n_(he) ⁰ and a step for determining mass attenuation coefficients, for each phase circulating in the multiphase fluid, at each energy k.

To that end, three monophase samples of gaseous phase, oily liquid phase, and aqueous liquid phase, respectively, are successively arranged in the pipe 14 and are measured. The density ρ_(φ) of each phase is also evaluated and the mass attenuation coefficients μ_(k,φ) are calculated to deduce the lineic coefficients therefrom using the equation: λ_(k,φ,i)=ρ_(φ,i)μ_(k,φ)  (0)

The measuring phase 102 includes a step 104 for continuous emission of the beams 36 and 38 at each energy le or he through the fluid 12 and for measuring the gamma photons received by the detector 32 at each measuring moment during the given measuring period T₁.

The measuring phase then comprises a step 108 for dividing the measuring period T₁ into a plurality of intervals I(1), . . . , I(n_(t)) in which each piece of representative information Γ_(g,i), W_(i) will be considered constant. The measuring phase then comprises step 110 for iterative adjustment of the value of each piece of representative information Γ_(g,i), W_(i), for each interval.

Step 110 includes, in reference to FIG. 6, a first phase for determining each piece of representative information Γ_(g,i), W_(i) in each interval I by successive iterations until a convergence criteria is verified without blocking of the determined values then, in reference to FIG. 7, a second phase of determining Γ_(g,i), W_(i) through successive iterations by blocking the values coming out of a physical measuring interval of these values and by transferring the corresponding photon counts.

In the measuring step, the photons transmitted through the fluid 12 are collected on the detector 32. A number of photon counts n_(he,p), n_(le,p) of high and low energy, respectively, is collected on the receiver 32 at the measuring pitch p, at an example frequency in the vicinity of 50 Hz.

In reference to FIG. 5, in the division step, the beginning 120 of a first measuring interval I_(k) is defined by measuring the number of counts n_(he,0), n_(le,0) done by the detector 34 at an initial measuring moment t₀ shown in FIG. 4.

Then, at each measuring moment t_(r) after the initial moment t₀, the expectation, variance, and covariance of the numbers of measured counts at the pitch p between the initial measuring moment t₀ and the measuring moment t_(r) are calculated, at each energy k, by the formulas:

$\begin{matrix} {E_{k} = {\frac{1}{N_{q}}{\sum\limits_{q = 0}^{r}n_{k,q}}}} & (1) \\ {V_{k} = {\frac{1}{N_{q} - 1}{\sum\limits_{q = 0}^{r}\left( {n_{k,q} - E_{k}} \right)^{2}}}} & (2) \\ {C = {\frac{1}{N_{q} - 1}{\sum\limits_{q = 0}^{r}{\left( {n_{{le},q} - E_{le}} \right) \cdot \left( {n_{{he},q} - E_{he}} \right)}}}} & (3) \end{matrix}$

-   -   where E_(k), V_(k) and C are the expectation, variance and         covariance, respectively, of the counts measured at energy k,         and n_(k,q) is the number of counts of energy k collected at the         measuring moment t_(q), N_(q) being the number of measuring         moments t₀, . . . t_(r) between the initial moment t₀ and the         measuring moment t_(r).

These statistical magnitudes V_(k) and C obtained exclusively based on the measured numbers of counts are compared to the theoretical statistical magnitudes obtained by considering a statistical law, advantageously a Poisson law, which would be obtained when the flow is stable, using the following equations:

$\begin{matrix} {V_{k}^{0} = \frac{E_{k}}{p}} & (4) \\ {C_{k}^{0} = 0} & (5) \end{matrix}$

where p is the sampling period separating two consecutive measuring moments.

The deviation between these differences is adimensioned by calculating the standard deviations of the variance and the covariance, assuming that they follow a law of χ² by the equations:

$\begin{matrix} {\sigma_{V_{k}} = {\left( \frac{E_{k}}{p} \right) \cdot \sqrt{\frac{2}{N_{q} - 1}}}} & (6) \\ {\sigma_{C} = {\left( \frac{1}{p} \right) \cdot \sqrt{\frac{E_{le} \cdot E_{he}}{N_{q} - 1}}}} & (7) \end{matrix}$

Then, an indicator χ representative of the difference between the measured statistics of the numbers of counts and the theoretical statistics obtained from the Poisson law is calculated using the equation:

$\begin{matrix} {\chi^{2} = {\frac{1}{3}\left\lbrack {\left( \frac{V_{le} - V_{le}^{0}}{\sigma_{Vle}} \right)^{2} + \left( \frac{V_{he} - V_{he}^{0}}{\sigma_{Vhe}} \right)^{2} + \left( \frac{C}{\sigma_{C}} \right)^{2}} \right\rbrack}} & (8) \end{matrix}$

If this indicator χ is below a predetermined value ε, for example equal to 1, the flow is still considered stable, and the preceding steps are repeated considering the numbers of counts between the initial measuring moment and the measuring moment t_(r+1) following the moment t_(r).

If this indicator χ is above a predetermined value ε, the flow is considered unstable.

The measuring moment t_(r) then constitutes the beginning of a second measuring interval I_(k+1) that is determined iteratively, as described for the first interval I_(k).

As illustrated by FIG. 4, it is thus possible to determine the different intervals I_(Γ1), . . . , I_(Γk) for which the flow is considered stable. In each interval I_(Γ1), . . . , I_(Γk), the first piece of representative information Γ_(g,i) is considered constant.

Likewise, a plurality of intervals I_(W1), . . . , I_(WI) in which the second piece of representative information W_(i) is considered constant is determined by a calculation or is fixed empirically.

Once the intervals I_(Γ1), . . . , I_(Γk), and I_(W1), . . . , I_(WI) are set for each of the pieces of representative information Γ_(g,i) and W_(i), the measuring period T₁ is divided into a plurality of intervals I₁, . . . , I_(nt) so that in each interval I₁, . . . , I_(nt), each piece of representative information Γ_(g,i) and W_(i) is assumed to be constant.

As illustrated by FIG. 4, tables Z _(Γ) and Z _(W) connecting each of the intervals I₁, . . . , I_(nt) and the values Γ_(g,i) and W_(i) on which they depend are built.

Moreover, for each energy k, the number of counts n_(k)(i) measured by the detector 32 in each interval I_(i) among the intervals I₁, . . . , I_(nt) is calculated by averaging the number of counts n_(k,q) measured at each measuring moment t_(r) in the interval I_(i).

The duration Δt_(i) of each interval I_(i) is therefore variable and is stored in a vector Δt _(i). Likewise, the values of the numbers of counts n_(le,i), n_(he,i) measured at high energy and low energy, respectively, in each interval i are stored in a matrix n with dimensions n_(t)×2.

Lineic attenuation coefficients λ_(g,k,i); λ_(o,k,t); λ_(w,k,i) are calculated for each phase and for each energy k in each interval I_(i), based on the coefficients μ_(k,φ) and densities ρ_(φ) and are stored in a matrix Λ with dimensions n_(t)×6.

Then, in reference to FIG. 6, a step for determining each piece of representative information Γ_(g,i) and W_(i) in each interval I_(Γ1), . . . , I_(Γk), and I_(W1), . . . , I_(WI) is done by successive iterations until a convergence criterion is verified, without blocking of the obtained values of Γ_(g,i) and W_(i).

To that end, in the initialization step 130, a vector x _(init)=[Γ_(g,i) (init), . . . , Γ_(g,k)(init), W₁(init), . . . W_(I)(init)] is initially defined.

Then, a series of iterations m, designated by reference 132, is done to determine the value of the vector x _(free)=[Γ_(g,i)(free), Γ_(g,k)(free), W₀(free), W_(I)(free)] in each interval I_(Γ1), . . . , I_(Γk), and I_(W1), . . . , I_(WI) when a convergence is obtained without blocking the values.

Each iteration 132 comprises the calculation 134 for each interval I₁, . . . , I_(nt) of the estimated numbers of counts measured at high energy and low energy {circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i) based on the first information Γ_(g,i)(m), W_(i) (m) obtained in the preceding iteration and a Beer-Lambert law, using the equation:

$\begin{matrix} {{\hat{n}}_{k,i} = {{n_{k}^{0} \cdot {\exp\left( {{- d} \cdot {\sum\limits_{{\varphi = g},w,o}^{\;}{\lambda_{k,\varphi,i}{\Gamma_{g,i}(m)}}}} \right)}} = {n_{k}^{0} \cdot {\exp\left( {{- d} \cdot \lambda_{k,i}} \right)}}}} & (9) \end{matrix}$

-   -   where d is the diameter of the pipe, λ_(k, φ,i) is the lineic         attenuation coefficient of the phase at energy k during the         interval I_(i).

Likewise, each iteration m comprises the calculation for each interval I₁, . . . , I_(nt) of the estimated numbers of counts in an empty pipe {circumflex over (n)}_(le,i) ⁰, {circumflex over (n)}_(he,i) ⁰ at high energy and low energy for each measuring interval i based on first information Γ_(g,i)(m), W_(i)(m) obtained in the preceding iteration and from a Beer-Lambert law, using the equation:

$\begin{matrix} {{\hat{n}}_{k,i}^{0} = {{n_{k,i} \cdot {\exp\left( {{+ d} \cdot {\sum\limits_{{\varphi = g},w,o}^{\;}{\lambda_{k,\varphi,i}{\Gamma_{g,i}(m)}}}} \right)}} = {n_{k,i} \cdot {\exp\left( {{+ d} \cdot \lambda_{k,i}} \right)}}}} & (10) \end{matrix}$

For each measuring interval I_(I) between I₁ and I_(nt), the values of Γ_(g,i)(m) and W_(i)(m) are extracted from the vector x(m) using correspondence tables Z _(Γ) and Z _(W).

Then the aqueous and oily phase fractions are calculated using the equations Γ_(w,i)=(1−Γ_(g,i))·W _(i)  (11) Γ_(o,i)=(1−Γ_(g,i))·(1−W _(i))  (12)

Then, the global lineic attenuation coefficients are calculated for each interval I_(i) at each energy using the equations: λ_(le,i)=Λ(i,1)·Γ_(g,i)+Λ(i,2)·Γ_(w,i)+Λ(i,3)·Γo,i  (13) λ_(he,i)=Λ(i,4)·Γ_(g,i)+Λ(i,5)·Γ_(w,i)+Λ(i,6)·Γo,i  (13)

This being done, the estimated numbers of measured counts {circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i) bet at low energy and high energy, respectively, are calculated using equation (9) above for each interval I_(i) and vectors {circumflex over (n)}_(le), {circumflex over (n)}_(he) with dimensions n_(t) are obtained.

Likewise, the estimated numbers of counts in an empty pipe {circumflex over (n)}_(le,i) ⁰, {circumflex over (n)}_(he,i) ⁰ at high energy and low energy for each measuring interval I_(i) are calculated based on equation (10) above and vectors {circumflex over (n)}_(le) ⁰, {circumflex over (n)}_(he) ⁰ with dimension n_(t) are obtained.

Then, in step 136, a residual L0 is determined as a function of a first criterion C₁ calculated from probabilities P1_(k,i)=P[n_(k,1)|{circumflex over (n)}_(k,i)(x)], according to a given statistical law, to measure, for each energy, the number of measured counts n_(le,i), n_(he,i) at each interval I_(i), the given statistical law being parameterized based on the number of estimated counts {circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i) at each interval, based on the values Γ_(g,i) and W_(i) in each interval.

Advantageously, the statistical law is a Poisson law.

To do this, the first criterion C₁ is obtained by computing, for each energy, the probability P1_(k,i) at each moment i using the following formula:

$\begin{matrix} {{P\; 1_{k,i}} = \frac{{\exp\left( {{- \hat{N}}1} \right)} \times \left( {\hat{N}1} \right)^{N\; 1}}{\Gamma\left( {{N\; 1} + 1} \right)}} & (15) \end{matrix}$

Γ being the gamma function, where N1 is obtained by making the scalar product of the vectors n _(k) and Δt _(i) and {circumflex over (N)}1 obtained by making the scalar product of vectors {circumflex over (n)}_(k) and Δt _(i).

Then, the first criterion C₁ is calculated using the equation:

$\begin{matrix} {C_{1} = {{f_{le} + f_{he}} = {{\sum\limits_{{k = {le}},{he}}^{\;}{- {\sum\limits_{i = 1}^{n_{t}}{\log\left\{ {P\left\lbrack n_{k,i} \middle| {{\hat{n}}_{k,i}\left( \underset{\_}{x} \right)} \right\rbrack} \right\}}}}} = {\sum\limits_{{k = {le}},{he}}^{\;}{- {\sum\limits_{i = 1}^{n_{t}}{\log\; P\; 1_{k,i}}}}}}}} & (16) \end{matrix}$

In this calculation of a maximum likelihood, it is equivalent to making the product of the probabilities P1_(k,i) or minimizing the sum of the terms −log P1_(k,i).

According to the present disclosure, the residual L0 also comprises a second criterion C₂ that is calculated from probabilities using a given statistical law to measure, for each energy, an average number of counts, the given statistical law being parameterized based on the average of the same estimated number of counts for each interval I_(i), based on the values Γ_(g,i) and W_(i) in each interval.

The average number of counts is advantageously the number n_(k) ⁰ of counts in an empty pipe measured during the calibration step for each energy k. The number of estimated counts for each interval I_(i) is given by the calculation of {circumflex over (n)}_(le,i) ⁰, {circumflex over (n)}_(he,i) ⁰ using equation (10).

To calculate the second criterion C₂, the average Ê_(k) ⁰ of the number of counts in an empty pipe estimated for each energy k is calculated based on the following equation:

$\begin{matrix} {{\hat{E}}_{k}^{0} = {\left( \frac{1}{T} \right) \cdot \left( {\sum\limits_{i = 1}^{nt}{{{\hat{n}}_{k,i}^{0} \cdot \Delta}\; t_{i}}} \right)}} & (17) \\ {{{where}\mspace{14mu} T} = \left( {\sum\limits_{i = 1}^{nt}{\Delta\; t_{i}}} \right)} & (18) \end{matrix}$

Then, for each energy, the probability

${P\; 2_{k}} = {P\left\lbrack {n_{k}^{0}\overset{\_}{\left| \;{{\hat{n}}_{k,i}^{0}\left( \underset{\_}{x} \right)} \right.}} \right\rbrack}$ of measuring n_(k) ⁰ numbers of counts in an empty pipe at energy k is calculated, according to a given statistical law knowing that the value of the average is estimated at Ê_(k) ⁰, the average Ê_(k) ⁰ being calculated based on the representative information Γ_(g,i) and W_(i) using equations (10) to (14) above.

The statistical law is advantageously the Poisson Law. The probability P2 is then calculated using the equation:

$\begin{matrix} {{P\; 2_{k}} = \frac{{\exp\left( {{- \hat{N}}2} \right)} \times \left( {\hat{N}2} \right)^{N\; 2}}{\Gamma\left( {{N\; 2}\; + 1} \right)}} & (19) \\ {{{where}\mspace{14mu} N\; 2} = {{n_{k}^{0} \times T\mspace{14mu}{and}\mspace{14mu}\hat{N}2} = {{\hat{E}}_{k}^{0} \times T}}} & (20) \end{matrix}$

The second criterion C₂ is the calculated using the equation:

$\begin{matrix} {{C\; 2} = {{\omega \cdot \left( {m_{le} + m_{he}} \right)} = {{\omega \cdot \left\lbrack {\sum\limits_{{le},{he}}{- {\log\left( {P\; 2_{k}} \right)}}} \right\rbrack} = {\omega \cdot \left\lbrack {\sum\limits_{{le},{he}}{- {\log\left( {P\left\lbrack n_{k}^{0} \middle| {\overset{\_}{{\hat{n}}_{k,i}^{0}}\left( \underset{\_}{x} \right)} \right\rbrack} \right)}}} \right\rbrack}}}} & (21) \end{matrix}$

where ω is a weight coefficient, for example equal to the square of n_(t).

Then, the residual L0 is calculated using the following formula: L0=C ₁ +C ₂ =f _(le) +f _(he)+ω·(m _(le) +m _(he))  (22)

In the following step 138 of the iteration m, a vectoral increment Δx is calculated to come closer to the convergence by causing a decrease of the residual L1, to reach a maximum likelihood between, on one hand, the measured values n_(k,i), and the estimated values

n̂_(k, i), and between, on the other hand, the average value n_(k) ⁰ and the average of the estimated values

$\overset{\_}{\;{{\hat{n}}_{k,i}^{0}\left( \underset{\_}{x} \right)}}.$

The increment Δx can be calculated by a variety of numerical methods. In this example, the Newton method can be used by calculating the Hessian matrix (second derivative) H and the gradient g (first derivative) of the residual L1 at the point under consideration x.

The matrix system H·Δx=−g is then resolved.

In practice, it is possible to simplify the Poisson law of probability with a normal law and to calculate the residual using the Gauss-Newton method. These methods are known by those skilled in the art.

Once the increment is calculated, a new value of each piece of representative information Γ_(g,i) (m+1) and W_(i)(m+1) is obtained using the formula: x (m+1)=[Γ_(g,1)(m+1), . . . ,Γ_(g,k)(m+1),W ₁(m+1), . . . ,W _(I)(m+1)]= x (m)+Δ x   (23)

The iterations m are repeated until a convergence criterion is verified in step 140.

In this example, the convergence criterion is for example calculated by the maximum absolute value Vmax of each term of the increment Δx. If Vmax is greater than a given value, for example less than 10⁻⁶, a new iteration m is done, while if Vmax is less than ε, the convergence criterion is met.

Alternatively, the convergence criterion can be determined by a stop criterion on the gradient. In step 142, when the convergence criterion is met, the matrix x _(free)=[Γ_(g,1)(free), . . . , Γ_(g,k) (free), W₀(free), . . . W₁(free)] is obtained.

In reference to FIG. 7, this matrix is processed so that each value Γ_(g,i), W_(i) is included in an interval of possible physical measurements [a, b], which in this example is between 0 and 1.

To that end, a series of iterations 150 is done, as long as there is still a value of x _(i) situated outside the interval of physical measurements [a, b].

Upon each iteration 150, each value x _(i) is scanned in step 152 to verify whether that value is below the minimum boundary a of the interval of physical measurements or is greater than the maximum boundary b of the interval of physical measurements.

In a first alternative, all of the values of x _(i) of the vector x of the representative information Γ_(g,i) and W_(i) that exceed a boundary a, b of the physical measurement interval are attributed the value x_(L)=a or b of the boundary, and all of the variables x_(L) are blocked at that value x_(L), so that they keep that value until the convergence.

In a second alternative, a first group of values x _(i) exceeding a boundary a, b of the physical measurement interval is assigned the value x_(L) of the corresponding boundary, and all of the variables of the first group are blocked at the value X_(L) until the convergence. On the contrary, a second group of values x _(i) exceeding a boundary a, b are kept at their initial values and these variables are left free during different iterations.

Thus, in one example, the value x _(i) exceeding a boundary, creating the greatest transfer of counts, is determined. This value x_(i) is then set at the value x_(L) of the boundary and is blocked until the convergence.

The count transfer resulting from the blockage of the variable x _(i) is determined.

To that end, a first residual L1 is calculated for the vector x _(free) obtained before blockage of the variable x _(i) using equation (22) above.

Then, a second residual L2 is calculated using equation (22) based on the vector x_(b) equal to x with the exception of the variable x_(b,i), which was blocked at the value x_(L).

The absolute value ΔL=|L2−L1| of the difference between the residuals L1 and L2 is then evaluated for each value x _(i) exceeding a boundary a, b and the value creating the most count transfers, i.e. the biggest ΔL, is kept constant for the continuation at the value x_(L).

A new vector x _(b) with at least one blocked value is then defined in step 154. The vector x _(b) is then adjusted by iterations 156 to reach a maximum likelihood with the vector x _(free) obtained at the end of the optimization without blockage in step 142.

This adjustment is done using the calculation, in step 160, of a residual with blockage L3 comprising the second criterion C₂ defined in equation (21). The residual L3 also includes a third criterion C₃ calculated from the probabilities P3_(k,i)=P[{circumflex over (n)}_(k,i)(x _(free))|{circumflex over (n)}_(k,i) (x _(b))], according to a given statistical law, to measure, for each energy k and in each interval I_(i), the number of estimated counts {circumflex over (n)}_(k,i)(x _(free)) for the vector x _(free) obtained by the optimization without blocking, the given statistical law being parameterized based on the number of estimated counts {circumflex over (n)}_(k,i)(x _(b)) for the vector blocked at each interval during each iteration 150.

The third criterion C3 is calculated using the following equation:

${C\; 3} = {{\sum\limits_{{le},{he}}{g_{k}\left( {{\underset{\_}{x}}_{free},{\underset{\_}{x}}_{b}} \right)}} = {\sum\limits_{{le},{he}}\left( {- {\sum\limits_{i = 1}^{nt}{\omega_{k,i}^{\prime}{\log\left( {P\left\lbrack {{\hat{n}}_{k,i}\left( {\underset{\_}{x}}_{free} \right)} \middle| {{\hat{n}}_{k,i}\left( {\underset{\_}{x}}_{b} \right)} \right\rbrack} \right)}}}} \right.}}$

where ω′_(k,i) is a weight calculated from the probability of the count transfer resulting from blocking at least one variable of the vector x _(b).

The probability P3_(k,i)=P[{circumflex over (n)}_(k,i)(x _(free))|{circumflex over (n)}_(k,i) (x _(b))] is calculated based on the Poisson law as described above.

To calculate the weight ω′_(k,i) resulting from blocking the variable x_(L), one determines whether the variable x_(L) constitutes a first piece of representative information Γ_(g,i), or a second piece of representative information W_(i).

In the first case, a vector x _(ref) is built by blocking all of the values of Γ_(g,i) at the blocked value x_(L). In the second case, the vector x _(ref) is built by blocking all of the values of W_(i) at the blocked value x_(L).

Then, in step 158, the estimated count numbers {circumflex over (n)}_(k,i)(x _(ref)) are calculated based on the vector x _(ref) at each moment I_(i) for each energy k using equations (9) and (11) to (14).

Then, the coefficients ω′_(k,i) are calculated by determining the probability P4_(k,i)=P[{circumflex over (n)}_(k,i)(x _(free))|{circumflex over (n)}_(k,i) (x _(b))] using the Poisson Law as previously described, using the equation:

$\begin{matrix} {{P\; 4_{k,i}} = \frac{{\exp\left( {{- \hat{N}}4} \right)} \times \left( {\hat{N}4} \right)^{N\; 4}}{\Gamma\left( {{N\; 4} + 1} \right)}} & (25) \end{matrix}$

where N4 is obtained by making the scalar product of the vectors {circumflex over (n)}_(k,i)(x _(ref)) and Δt _(i) and {circumflex over (N)}4 is obtained by making the scalar product of the vectors {circumflex over (n)}_(k)(x _(b)) and Δt _(i).

Then, ω′_(k,i) is calculated using the equation:

$\begin{matrix} {\omega_{k,i}^{\prime} = \left( {P\; 3_{k,i}} \right)^{\frac{1}{2}}} & (26) \end{matrix}$

The residual L3 is calculated using the equation: L3=C ₂ +C ₃  (27)

Then, in step 162, an increment Δx is calculated to decrease the residual L3, as previously described in step 138, to reach a maximum likelihood between, on one hand, the estimated values {circumflex over (n)}_(k,i)(x _(free)) obtained without blocking, and the estimated values {circumflex over (n)}_(k,i)(x _(b)) obtained after blocking, and between, on the other hand, the average value n_(k) ⁰ and the average of the estimated values

$\overset{\_}{\;{{\hat{n}}_{k,i}^{0}\left( \underset{\_}{x} \right)}}.$

The iterations are repeated as long as the convergence criterion of step 164 has not been met.

This criterion is advantageously equal to the criterion defined in step 140.

Once the convergence criterion is met, and once all of the values x_(b,i), of the vector x _(b) obtained after blocking are included in the interval [a, b] of physical values, the vector x _(b), end obtained describes all of the values Γ_(g,i) and W_(i) on the different measuring intervals of the measuring period T₁. Another measuring period can then be processed.

In the disclosure hereof, the optimization done by using the second criterion C₂ significantly improves the precision of the results obtained on the calculation of the representative information Γ_(g,i) and W_(i), in particular when the gas content is high.

Blocking physically incorrect values of Γ_(g,i) and W_(i) and the weighted transfer of the count numbers having led to those values makes it possible to offset the errors naturally observed, without introducing a significant statistical bias.

It is thus possible to obtain great precision while also keeping excellent measurement dynamics.

The methods used are numerically easy to resolve and more reliable than using a direct resolution method.

Dividing the measuring period T₁ into a plurality of intervals I_(i) using an adaptive method greatly simplifies the numerical resolution of the problem posed by limiting the number of statistical values with a minimal risk of not capturing variations of the flow.

The division method is based solely on the statistics of the number of measured counts, which makes it particularly simple to carry out. 

The invention claimed is:
 1. A method for determining at least one piece of information (Γ_(g), W) representative of a phase fraction of a multiphase fluid circulating in a pipe, the method comprising the steps of: (a) emitting a number of gamma photon counts at at least one energy through the fluid, and (b) measuring a number of counts (n_(le), n_(he)) received for each energy, after passage in the fluid; (c) dividing a given measuring period (T₁) into a plurality of measuring intervals (I_(i)) in which each piece of representative information (Γ_(g,i), W_(i)) is assumed to be constant; (d) choosing an initial value of the or each piece of representative information (Γ_(g,i), W_(i)) in each interval; (e) determining the or each piece of representative information (Γ_(g,i), W_(i)) in each interval (I_(i)) through successive iterations until a convergence criterion is verified, each iteration comprising: (e₁) estimating a number of counts received ({circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i)) at each energy in each interval (I_(i)) based on the or each piece of representative information (Γ_(g,i), W_(i)) determined at a preceding iteration, (e₂) calculating a residual (L₀) comprising a first criterion (C₁) calculated from probabilities according to a given statistical law to measure, for each energy, the number of counts (n_(le,i), n_(he,i)) measured at each interval, the given statistical law being parameterized based on the number of counts estimated ({circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i)) in step (e₁) at each interval (I_(i)); (e₃) determining new values of the or each piece of representative information (Γ_(g,i), W_(i)) to minimize the residual (L₀); step (e) including, at each iteration, (e₄) calculating the average (Ê_(k) ⁰), for each energy, of an estimated number of counts ({circumflex over (n)}_(k) ⁰) for each interval (I_(i)) based on the representative information (Γ_(g,i), W_(i)) determined in a preceding iteration, the residual (L₀) calculated in step (e₂) comprising a second criterion (C₂) calculated from probabilities according to a given statistical law to measure, for each energy, an average number of counts (n_(k) ⁰), the given statistical law being parameterized based on the average (Ê_(k) ⁰) calculated in step (e₄).
 2. The method according to claim 1, wherein for each energy, the average count number is an average number of counts in an empty pipe (n_(k) ⁰) measured during a calibration step at each energy, in the absence of fluid in the pipe, the step (e₄) including the calculation of the average (Ê_(k) ⁰) for each energy, of the number of counts in an empty pipe ({circumflex over (n)}_(k) ⁰) estimated for each interval based on the representative information (Γ_(g,i), W_(i)) determined in a preceding iteration.
 3. The method according to claim 1, wherein the representative information (Γ_(g,i), W_(i)) is respectively the lineic fraction of gas (Γ_(g,i)) according to a diameter of the pipe and the volume fraction of water (W_(i)) in the liquid phase present in the multiphase fluid.
 4. The method according to claim 1, wherein the statistical law is a Poisson law.
 5. The method according to claim 1, wherein the residual (L) is calculated using the formula: f _(le) +f _(he)+ω·(m _(le) +m _(he)) in which f_(le) and f_(he) are the first criteria (C₁) calculated at a first energy and a second energy, respectively, m_(le) and m_(he) are the second criteria (C₂) calculated at the first energy and the second energy, respectively, and ω is a weight coefficient.
 6. The method of claim 1, further comprising scanning the raw representative information (Γ_(g,i)(free), W_(i)(free)) obtained at the end of step (e) at each interval (I_(i)) to determine the raw representative information coming from a given interval of physical values ([a, b]), then blocking at least one piece of raw representative information (x_(i)) coming out of the interval of physical values so that its value (x_(L)) remains equal to one end of the interval, the method then comprising a step (f) for determining each piece of representative information (Γ_(g,i), W_(i)) in each interval through successive iterations until verification of a convergence criterion, each iteration comprising: (f₁) estimating a number of counts ({circumflex over (n)}_(k,i)(x _(b))) received at each energy in each interval based on the representative information determined at a preceding iteration from representative information obtained after blocking the or each piece of locked representative information; (f₂) calculating a residual (L₃) comprising a third criterion (C₃) calculated from probabilities according to a given statistical law to measure, for each energy, the number of estimated counts ({circumflex over (n)}_(k,i)(x _(free))) at each interval from representative information obtained without blocking at the end of step (e), the given statistical law being parameterized based on the number of counts {circumflex over (n)}_(k,i)(x _(b)) estimated in step (f₁) at each interval.
 7. The method according to claim 6, wherein the blocking step includes the selection of at least a first group of representative information (Γ_(g,i), W_(i)) having a value situated outside the interval of physical values given, without locking a second group of determined representative information having a value situated outside the interval of given physical values, then carrying out step (f).
 8. The method according to claim 6, wherein the or each piece of blocked representative information is chosen based on a criterion (ΔL) representative of the count transfer resulting from locking the representative information.
 9. The method according to claim 6, wherein the step of calculating the third criterion (C₃) comprises calculating a count transfer probability coefficient [ω′_(k,i)] resulting from blocking at least one piece of representative information in each interval at each energy to weight each probability calculated in step (f₂).
 10. The method according to claim 1, wherein the step for dividing the given period into a plurality of intervals comprises: (c₁) for each interval, determining a beginning of an interval (t₀), then calculating, for successive measuring moments (t_(r)) moving away from the beginning of the interval (t₀), at least one difference between at least one statistical magnitude (V_(k), C) calculated from the number of counts measured between the beginning of the interval (t₀) and the measuring moment (t_(r)) after the beginning of the interval (t₀) and the same statistical magnitude (V_(k) ⁰, C⁰) calculated based on a known statistical law, until a criterion determined based on said difference is greater than a determined value, the measuring moment in progress then constituting the beginning of another interval, (c₂) calculating the average of the number of counts (n_(k,q)) measured at each energy at each measuring moment (t_(r)) in the interval, the average of the number of measured counts defining the number of counts (n_(k,i)) measured in the interval; (c₃) repeating steps (c₁) and (c₂) to define the other interval from the moment following the end of the interval.
 11. The method according to claim 10, wherein the statistical magnitude comprises the variance (V_(k)) and/or covariance (C) of the number of counts (n_(k,q)) measured at each measuring moment between the beginning of the interval and the subsequent measuring moment.
 12. The method of claim 10, wherein the statistical law is a Poisson law, the statistical magnitude being calculated from the Poisson law.
 13. The method according to claim 12, wherein the step of determining the criterion comprises calculating at least the difference $\left\lbrack \frac{V_{k} - V_{k}^{0}}{\sigma_{V_{k}}} \right\rbrack,$ where k is the energy, V_(k) is the variance of the number of measured counts between the beginning of the interval and the measuring moment (t_(r)) after the beginning of the interval, V_(k) ⁰ is the variance calculated from the statistical law, and σ_(Vk) is the standard deviation calculated from the statistical law.
 14. A method for determining at least one piece of information (Γ_(g), W) representative of a phase fraction of a multiphase fluid circulating in a pipe, the method comprising the steps of: (f) emitting a number of gamma photon counts at least one energy through the fluid, and (g) measuring a number of counts (n_(le), n_(he)) received for each energy, after passage in the fluid; (h) dividing a given measuring period (T₁) into a plurality of measuring intervals (I_(i)) in which each piece of representative information (Γ_(g,i), W_(i)) is assumed to be constant; (i) choosing an initial value of the or each piece of representative information (Γ_(g,i), W_(i)) in each interval; (j) determining the or each piece of representative information (Γ_(g,i), W_(i)) in each interval (I_(i)) through successive iterations until a convergence criterion is verified, each iteration comprising: (e₁) estimating a number of counts received ({circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i)) at each energy in each interval (I_(i)) based on the or each piece of representative information (Γ_(g,i), W_(i)) determined at a preceding iteration, (e₂) calculating a residual (L₀) comprising a first criterion (C₁) calculated from probabilities according to a given statistical law to measure, for each energy, the number of counts (n_(le,i), n_(he,i)) measured at each interval, the given statistical law being parameterized based on the number of counts estimated ({circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i)) in step (e₁) at each interval (I_(i)); (e₃) determining new values of the or each piece of representative information (Γ_(g,i), W_(i)) to minimize the residual (L₀); step (e) including, at each iteration, (e₄) calculating the average (Ê_(k) ⁰), for each energy, of an estimated number of counts ({circumflex over (n)}_(k) ⁰) for each interval (I_(i)) based on the representative information (Γ_(g,i), W_(i)) determined in a preceding iteration, the residual (L₀) calculated in step (e₂) comprising a second criterion (C₂) calculated from probabilities according to a given statistical law to measure, for each energy, an average number of counts (n_(k) ⁰) the given statistical law being parameterized based on the average (Ê_(k) ⁰) calculated in step (e₄); and further comprising scanning the raw representative information (Γ_(g,i)(free), W_(i)(free)) obtained at the end of step (e) at each interval (I_(i)) to determine the raw representative information coming from a given interval of physical values ([a, b]), then blocking at least one piece of raw representative information (x_(i)) coming out of the interval of physical values so that its value (x_(L)) remains equal to one end of the interval, the method then comprising a step (f) for determining each piece of representative information (Γ_(g,i), W_(i)) in each interval through successive iterations until verification of a convergence criterion, each iteration comprising: (f₁) estimating a number of counts ({circumflex over (n)}_(k,i)(x _(b))) received at each energy in each interval based on the representative information determined at a preceding iteration from representative information obtained after blocking the or each piece of locked representative information; (f₂) calculating a residual (L₃) comprising a third criterion (C₃) calculated from probabilities according to a given statistical law to measure, for each energy, the number of estimated counts ({circumflex over (n)}_(k,i)(x _(free))) at each interval from representative information obtained without blocking at the end of step (e), the given statistical law being parameterized based on the number of counts {circumflex over (n)}_(k,i)(x _(b)) estimated in step (f₁) at each interval.
 15. The method according to claim 14, wherein the blocking step includes the selection of at least a first group of representative information (Γ_(g,i), W_(i)) having a value situated outside the interval of physical values given, without locking a second group of determined representative information having a value situated outside the interval of given physical values, then carrying out step (f).
 16. The method according to claim 14, wherein the or each piece of blocked representative information is chosen based on a criterion (ΔL) representative of the count transfer resulting from locking the representative information.
 17. The method according to claim 14, wherein the step of calculating the third criterion (C₃) comprises calculating a count transfer probability coefficient [ω′_(k,i)] resulting from blocking at least one piece of representative information in each interval at each energy to weight each probability calculated in step (f₂).
 18. The method according to claim 14, wherein the step for dividing the given period into a plurality of intervals comprises: (c₁) for each interval, determining a beginning of an interval (t₀), then calculating, for successive measuring moments (t_(r)) moving away from the beginning of the interval (t₀), at least one difference between at least one statistical magnitude (V_(k), C) calculated from the number of counts measured between the beginning of the interval (t₀) and the measuring moment (t_(r)) after the beginning of the interval (t₀) and the same statistical magnitude (V_(k) ⁰, C⁰) calculated based on a known statistical law, until a criterion determined based on said difference is greater than a determined value, the measuring moment in progress then constituting the beginning of another interval, (c₂) calculating the average of the number of counts (n_(k,q)) measured at each energy at each measuring moment (t_(r)) in the interval, the average of the number of measured counts defining the number of counts (n_(k,i)) measured in the interval; (c₃) repeating steps (c₁) and (c₂) to define the other interval from the moment following the end of the interval.
 19. The method according to claim 18, wherein the statistical magnitude comprises the variance (V_(k)) and/or covariance (C) of the number of counts (n_(k,q)) measured at each measuring moment between the beginning of the interval and the subsequent measuring moment.
 20. The method of claim 18, wherein the statistical law is a Poisson law, the statistical magnitude being calculated from the Poisson law.
 21. A method for determining at least one piece of information (Γ_(g), W) representative of a phase fraction of a multiphase fluid circulating in a pipe, the method comprising the steps of: (k) emitting a number of gamma photon counts at least one energy through the fluid, and (l) measuring a number of counts (n_(le), n_(he)) received for each energy, after passage in the fluid; (m) dividing a given measuring period (T₁) into a plurality of measuring intervals (I_(i)) in which each piece of representative information (Γ_(g,i), W_(i)) is assumed to be constant; (n) choosing an initial value of the or each piece of representative information (Γ_(g,i), W_(i)) in each interval; (o) determining the or each piece of representative information (Γ_(g,i), W_(i)) in each interval (I_(i)) through successive iterations until a convergence criterion is verified, each iteration comprising: (e₁) estimating a number of counts received ({circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i)) at each energy in each interval (I_(i)) based on the or each piece of representative information (Γ_(g,i), W_(i)) determined at a preceding iteration, (e₂) calculating a residual (L₀) comprising a first criterion (C₁) calculated from probabilities according to Poisson law to measure, for each energy, the number of counts (n_(le,i), n_(he,i)) measured at each interval, the Poisson law being parameterized based on the number of counts estimated ({circumflex over (n)}_(le,i), {circumflex over (n)}_(he,i)) in step (e₁) at each interval (I_(i)); (e₃) determining new values of the or each piece of representative information (Γ_(g,i), W_(i)) to minimize the residual (L₀); step (e) including, at each iteration, (e₄) calculating the average (Ê_(k) ⁰), for each energy, of an estimated number of counts ({circumflex over (n)}_(k) ⁰) for each interval (I_(i)) based on the representative information (Γ_(g,i), W_(i)) determined in a preceding iteration, the residual (L₀) calculated in step (e₂) comprising a second criterion (C₂) calculated from probabilities according to a given statistical law to measure, for each energy, an average number of counts (n_(k) ⁰), the Poisson law being parameterized based on the average (Ê_(k) ⁰) calculated in step (e₄); and further comprising scanning the raw representative information (Γ_(g,i)(free), W_(i)(free)) obtained at the end of step (e) at each interval (I_(i)) to determine the raw representative information coming from a given interval of physical values ([a, b]), then blocking at least one piece of raw representative information (x_(i)) coming out of the interval of physical values so that its value (x_(L)) remains equal to one end of the interval, the method then comprising a step (f) for determining each piece of representative information (Γ_(g,i), W_(i)) in each interval through successive iterations until verification of a convergence criterion, each iteration comprising: (f₁) estimating a number of counts ({circumflex over (n)}_(k,i)(x _(b))) received at each energy in each interval based on the representative information determined at a preceding iteration from representative information obtained after blocking the or each piece of locked representative information; (f₂) calculating a residual (L₃) comprising a third criterion (C₃) calculated from probabilities according to Poisson law to measure, for each energy, the number of estimated counts ({circumflex over (n)}_(k,i)(x _(free))) at each interval from representative information obtained without blocking at the end of step (e), the Poisson law being parameterized based on the number of counts {circumflex over (n)}_(k,i)(x _(b)) estimated in step (f₁) at each interval. 